1. Field of the Invention
The invention relates to a device for mechanical coupling of an output-side converter part of an electromechanical hearing aid converter which can be implanted in an artificial mastoid cavity outside the region of the middle ear, the converter part having a capacity to be excited to mechanical vibrations, to a preselected coupling site on the ossicular chain, the footplate of the stapes or the membrane which closes the round window or an artificial window in the cochlea, in the vestibulum or in the labyrinth (equilibrium organ), with a biocompatible, mechanically passive coupling arrangement which is connected to the output-side converter part and which reaches in the implanted state from the mastoid cavity into the tympanic cavity and adjoins the coupling site with the coupling end which is away from the hearing aid converter.
2. Description of Related Art
Electronic measures for rehabilitation of inner ear damage which cannot be surgically corrected have attained an important position today. With total failure of the inner ear, cochlear implants with direct electrical stimulation of the remaining auditory nerves are routinely used in clinical practice. In moderate to severe inner ear damage, for the first time fully digital hearing aids are being used which open a new world of electronic audio signal processing and offer expanded possibilities of deliberate precision audiological matching of the hearing aids to the individual inner ear damage. In spite of the major improvements in hearing aid hardware which have been achieved in recent years, in conventional hearing aids fundamental defects remain; they are due to the principle of acoustic amplification, i.e., especially to the conversion of the electronically amplified signal back into airborne sound. These defects include aspects such as the visibility of the hearing aids, poor sound quality as a result of the electromagnetic converters (speakers), the closed external auditory passage and feedback effects with high acoustic amplification.
As a result of these fundamental defects, there has long been the desire to abandon conventional hearing aids with acoustic excitation of the damaged inner ear and to replace these devices by partially implantable or fully implantable hearing systems with direct mechanical stimulation. Implantable hearing systems differ from conventional hearing aids; admittedly, the acoustic signal is converted into an electrical signal with a converter (microphone) and is amplified in an electronic signal processing stage; this amplified electrical signal is however not supplied to an electroacoustic converter, but to an implanted electromechanical converter which produces output-side mechanical vibrations which are supplied directly, therefore with direct mechanical contact, to the middle ear or inner ear or indirectly by a force-fit via an air gap in electromagnetic converter systems, for example. This principle applies regardless of whether there has been a partial or complete implantation of all necessary system components and also regardless of whether pure labyrinthine deafness is to be rehabilitated with a completely intact middle ear or a combined deafness (middle ear and inner ear damaged). Therefore, in the more recent scientific literature and in numerous patents, implantable electromechanical converters and processes for direct coupling of the mechanical converter vibrations to the intact middle ear or to the inner ear for rehabilitation of pure labyrinthine deafness and also to the remaining ossicles of the middle ear in an artificially or pathologically altered middle ear for care of conductive deafness and their combinations have been described.
Basically, all physical conversion principles can be used as electromechanical converter processes, i.e., electromagnetic, electrodynamic, magnetostrictive, dielectric, and piezoelectric. In recent years, various research groups have focused essentially on two of these processes; electromagnetic and piezoelectric. An outline of these converter versions can be found in Zenner and Leysieffer (HNO 1997 Vol. 45, 749-774).
In the piezoelectric process, mechanically direct coupling of the output-side converter vibrations to the middle ear ossicle or directly to the oval window is necessary. In the electromagnetic principle, the force coupling can take place via an air gap (xe2x80x9ccontactlessxe2x80x9d), i.e., only one permanent magnet is placed by permanent fixation in direct mechanical contact with a middle ear ossicle On the other hand, it is possible to execute the converter entirely within a housing (the coil and the magnet being coupled with the smallest possible air gap) and to transfer the output-side vibrations via a mechanically stiff coupling element with direct contact to the middle ear ossicle (Leysieffer et al. 1997 (HNO 1997, Vol. 45. pp. 792-800).
The patent literature contains some of the aforementioned versions of both electromagnetic and also piezoelectric hearing aid converters: U.S. Pat. No. 5,707,338 (Adams et al.), WO 98/06235 (Adams et al.), WO 98/06238 (Adams et al.), WO 98/06236 (Kroll et al.), WO 98/06237 (Bushek et al.), U.S. Pat. No. 5,554,096 (Ball), U.S. Pat. No. 3,712,962 (Epley), U.S. Pat. No. 3,870,832 (Fredrickson), U.S. Pat. No. 5,277,694 (Leysieffer et al.), commonly owned U.S. patent application Ser. Nos. 09/275,872 and 09/311,563 (Leysieffer), U.S. Pat. No. 5,015,224 (Maniglia), U.S. Pat. No. 3,882,285 (Nunley), and U.S. Pat. No. 4,850,962 (Schaefer).
The partially implantable piezoelectric hearing system of the Japanese group Suzuki and Yanigahara presupposes for implantation of the converter the absence of the middle ear ossicles and a free tympanic cavity in order to be able to couple the piezoelement to the stapes (Yanigahara et al.: Efficacy of the partially implantable middle ear implant in middle and inner ear disorders. Adv. Audiol., Vol. 4, Karger Basel (1988), pp. 149-159; Suzuki et al.: Implantation of partially implantable middle ear implant and the indication. Adv. Audiol., Vol. 4, Karger Basel (1998), pp. 160-166). Likewise, in the process of a partially implantable hearing system for those suffering from labyrinthine deafness of U.S. Pat. No. 4,850,962 (Schaefer), basically, the incus is removed in order to be able to couple a piezoelectric converter element to the stapes. This also applies especially to other developments which are based on Schaefer technology and which are documented in the aforementioned patents (U.S. Pat. No. 5,707,338, WO 98/06235, WO 98/06238, WO 98/06236, and WO 98/06237).
The electromagnetic converter of Ball (xe2x80x9cFloating Mass Transducer FMTxe2x80x9d, U.S. Pat. No. 5,624,376 and U.S. Pat. No. 5,554,096) is conversely fixed with titanium clips directly on the long process of the incus when the middle ear is intact. The electromagnetic converter of the partially implantable system of FREDRICKSON (Fredrickson et al.: Ongoing investigations into an implantable electromagnetic hearing aid for moderate to severe sensorineural hearing loss. Otolaryngologic Clinics of North America, Vol. 28/1 (1995), pp. 107-121) is mechanically coupled directly to the body of the incus when the ossicular chain of the middle ear is likewise intact. The same applies to the piezoelectric and electromagnetic converters of LEYSIEFFER (Leysieffer et al.: An implantable piezoelectric hearing aid converter for patients with labyrinthine deafness. HNO 1997/45, pp. 792-800, U.S. Pat. No. 5,277,694, U.S. patent application Ser. No. 09/275,872, and U.S. patent application Ser. No. 09/311,563). Also in the electromagnetic converter system of MANIGLIA (Maniglia et al: Contactless semi-implantable electromagnetic middle ear device for the treatment of sensorineural hearing loss, Otolaryngologic Clinics of North America, Vol. 28/1 (1995), pp. 121-141) when the ossicular chain is intact a permanent magnet is permanently fixed mechanically to the ossicular chain and is however mechanically driven via an air gap coupling by a coil.
In the described converter and coupling versions basically two implantation principles can be distinguished:
a) In the case of the one principle the electromechanical converter with its active converter element is located itself in the middle ear region in the tympanic cavity and the converter is directly connected there to an ossicle or the inner ear (U.S. Pat. No. 4,850,962, U.S. Pat. No. 5,015,225, U.S. Pat. No. 5,707,338, WO 98/06235, WO 98/06238, WO 98/06236, WO 98/06237, U.S. Pat. No. 5,624,376, U.S. Pat. No. 5,554,096).
b) In the other principle the electromechanical converter with its active converter element is located outside of the middle ear region in an artificially formed mastoid cavity. The output-side mechanical vibrations are then transmitted to the middle or inner ear (Fredrickson et al.: Ongoing investigations into an implantable electromagnetic hearing aid for moderate to sever sensorineural hearing loss. Otolaryngologic Clinics of North America, Vol. 28/1 (1995), pp. 107-121; U.S. Pat. No. 5,277,694; U.S. patent application Ser. Nos. 09/275,872 and 09/311,563) by means of mechanically passive coupling elements via suitable surgical accesses (natural aditus ad antrum, opening of the chorda-facialis angle or via an artificial hole from the mastoid).
In version a), the converter can be made as a so-called xe2x80x9cfloating massxe2x80x9d converter, i.e., the converter elements does not require any xe2x80x9creactionxe2x80x9d via secure screwing to the skull bone, but it vibrates based on the laws of mass inertia with its converter housing and transmits this directly to a middle ear ossicle (U.S. Pat. No. 5,624,376, U.S. Pat. No. 5,554,096, U.S. Pat. No. 5,707,338, WO 98/06236). On the one hand, this means that an implantable fixation system on the cranial vault can be advantageously omitted, and on the other hand, this version disadvantageously means that bulky artificial elements must be placed in the tympanic cavity and their long term stability and bio-stability are currently not known or guaranteed especially in the case of temporary pathological changes of the middle ear (for example, otitis media). One major disadvantage lies in that the converter is moved out of the mastoid with its electrical supply line into the middle ear and must be fixed there using suitable surgical tools; this requires expanded access through the chorda facialis angle and thus entails a latent hazard to the facial nerve which is located in the immediate vicinity.
In the converter versions as per b), the converter housing with the implantable positioning and fixation systems must be attached to the cranial vault (advantageous embodiment published German Patent Application 196 18 964 corresponding to U.S. Pat. No. 5,788,711). One disadvantage of the versions as per b) is that a depression must be made in the target ossicle in order to be able to apply the coupling element. This, on the one hand, is technically complex and expensive and, on the other hand, entails risks to the patient. Both in the partially implantable system of FREDRICKSON (Ongoing investigations into an implantable electromagnetic hearing aid for moderate to severe sensorineural hearing loss, Otolaryngologic Clinics of North America, Vol. 28/1 (1995), pp. 107-121) as well as in the fully implantable hearing system of LEYSIEFFER and ZENNER (HNO 1998, vol. 46, 853-863 and 844-852), when the vibrating converter part is coupled to the body of the incus it is assumed for permanent and mechanically secure vibration transmission that the tip of the coupling rod which is placed in the laser-induced depression of the middle ear ossicle undergoes osseointegration over the long term, i.e., the coupling rod coalesces solidly with the ossicle, and thus, ensures reliable transmission of dynamic compressive and tensile forces. However, this long-term effect is currently not yet scientifically proven or certain. Furthermore, in this type of coupling, in case of a technical converter defect, there is the disadvantage that decoupling from the ossicle to remove the converter can only be done with mechanically based surgical methods; this can mean considerable hazard to the middle ear and especially the inner ear.
The major advantage of these converter embodiments as per b), however, is that the middle ear remains largely free and coupling access to the middle ear can take place without major possible hazard to the facial nerve. One preferable surgical process for this purpose is described in U.S. patent application Ser. No. 09/168,079. Basic advantageous forms of passive coupling elements for transmission of the output-side converter vibrations from the mastoid to the middle ear or inner ear are described in published European Patent Applications EP-A 0 499 940 (corresponding to U.S. Pat. No. 5,277,964), and EP-A 0 901 779 (corresponding to U.S. patent application Ser. No. 09/042,805) and in HNO 1998 Vol. 46, 27-37, Lehner et al.: xe2x80x9cCold-flowing elements for coupling of an implantable hearing aid converter to the auditory ossicle or perilymph.xe2x80x9d They are especially coupling elements of gold, preferably soft-annealed fine gold, in the form of a C-band for the long process of the incus, a band loop for the long process of the incus and a tiny bell for the head of the stapes, and these coupling elements can be coupled using instruments which are standard in ear surgery, and if necessary, they can also be detached again.
In an external (not implantable) hearing aid with an electromagnetic output converter (EP-B-0 556 300), keeping a permanent magnet of the output converter on the outer surface of the eardrum by means of manually detachable surface adhesion by non-invasive means is known. But in this case, problems such as a non-optimum form of vibration of the footplate of the stapes by restraint of the ossicle and risky work in the inner ear during implantation do not play a role. In addition the large area which is available on the outside surface of the eardrum is not comparable to the tiny coupling surfaces in the middle ear.
In addition, a passive ossicle prosthesis with a head, a shaft and a flexible intermediate piece is known (WO 90/11737). The intermediate piece makes it possible to adjust the angular alignment of the shaft with reference to the head. In the implanted prosthesis the shaft is supported on the arch of the stapes, one leg of the stapes or the footplate of the stapes, while the prosthesis head adjoins the eardrum or the malleus under prestress such that the eardrum is slightly tensioned. The compressive force which is exerted on the prosthesis by the eardrum as a result keeps the prosthesis in place The head and the shaft of the prosthesis are preferably made of hydroxyl apatite or a mixture of hydroxyl apatite particles and silicone or polyurethane. Human tissue adheres to this material, by which fixing of the prosthesis in the middle ear is to be supported.
The object of this invention is to devise a device for coupling of the hearing aid converter and for transmission of the output-side converter vibrations to the middle ear or inner ear, which can be applied more easily and reliably while preserving the aforementioned advantages of version b), which minimizes the necessary risky effort during implantation in the inner ear, which also facilitates decoupling which becomes necessary later under certain circumstances and which promotes an optimum form of vibration of the footplate of the stapes.
Proceeding from a device of the type which is known from U.S. Pat. No. 5,941,814 and HNO Vol. 46, 27-37, i.e., a device for mechanical coupling of an output-side converter part of an electromechanical hearing aid converter which can be implanted in an artificial mastoid cavity outside the region of the middle ear, the converter part having a capacity to be excited to mechanical vibrations, to a preselected coupling site on the ossicular chain, the footplate of the stapes or the membrane which closes the round window or an artificial window in the cochlea, in the vestibulum or in the labyrinth (equilibrium organ), with a biocompatible mechanically passive coupling arrangement which is connected to the output-side converter part and which reaches in the implanted state from the mastoid cavity into the tympanic cavity and adjoins the coupling site with the coupling end which is away from the hearing aid converter, this object is achieved in accordance with the present invention by forming the coupling end of a coupling element with a contact surface which has a surface shape which is matched or which can be matched to the surface shape of the coupling site and has a surface composition and surface size such that, by placing the coupling end against the coupling site, dynamic tension-compression force coupling of the coupling element and the preselcted coupling site occurs by surface adhesion which is sufficient for reliable mutual connection of the coupling element and the coupling site.
The device according to the invention provides for coupling of the output-side converter part of an electromechanical hearing aid converter which can be implanted in a mastoid cavity to an ossicle (malleus, incus, stapes; preferably the incus), to the footplate of the stapes or to a membrane which closes the round window or an artificial window by surface adhesion. Here the expression xe2x80x9csurface adhesionxe2x80x9d means adhesion under the influence of the molecular forces of attraction which take effect with a sufficiently close approach to the contact surfaces, or mutual mechanical interlocking of the contact surfaces without using an adhesive or cement. Air bubbles which are enclosed in the surface depressions can also contribute to corresponding surface adhesion. If the contact surfaces are exposed to forces which seek to separate the surfaces these air bubbles produce a suction effect; this makes separation very difficult (Bild der Wissenschaft, April 1999, p, 10).
One basic advantage of this adhesion coupling is that the coupling site, for example, of the ossicle is not xe2x80x9crestrainedxe2x80x9d primarily in the direction of vibration of the driving converter, and this xe2x80x9crestraintxe2x80x9d can lead to a non-optimum form of vibration of the footplate of the stapes in the oval window. (One preferable form of vibration is a piston-like vibration of the footplate of the stapes perpendicular to its plane). Rather, the ossicle adjusts its (frequency-dependent) vibration direction due to the dynamic properties of the intact middle ear itself when surface adhesion coupling is being used. This advantage also applies for a non-intact (partially) decomposed ossicular chain and coupling to the remainder of the chain facing the inner ear, and in the extreme case, also for only residual stapes or only the footplate of the stapes since it is suspended by the so-called ligament (an elastic annular band which xe2x80x9choldsxe2x80x9d the stapes in the oval window). In addition, there is postoperative detachability of the ossicle coupling even after years in the body. This approach to connection is possible with reasonable expenditure of force and using instruments which are standard in ear surgery.
In particular, the coupling arrangement feasibly has a coupling rod which reaches in the implanted state from the mastoid cavity into the tympanic cavity and which is securely joined to the output-side converter part and has a coupling element which is connected or can be connected to the end of the coupling rod away from the output-side converter part and forms the coupling end of the coupling arrangement. This coupling rod represents a mechanically stiff coupling element of comparatively small mass which can be securely attached to the outside of a vibratory membrane of the electromechanical hearing aid converter and which can be pushed into the middle ear from the mastoid when implanted through the natural or artificially, slightly widened aditus ad antrum, through an opening of the chorda facialis angle or through an artificial hole in order to be connected there via the coupling element to the desired coupling site. It is thus guaranteed that the vibratory stimulus of the converter is introduced into the coupling site, such as a target ossicle, largely without losses.
The coupling rod and the coupling element can be Joined to one another via a flexible intermediate element which can be a separate component or which can be formed by the coupling element itself In the latter case, the coupling element can be provided easily with a constriction for purposes of forming the flexible intermediate element. The flexible intermediate element can be optimally adjusted automatically with respect to its solid angle or can be individually plastically adjusted in the optimum manner by the surgeon during the surgery.
The flexible intermediate element can advantageously be made as a spring element and can be made of a metal alloy with memory effect or so-called xe2x80x9csuperelasticity,xe2x80x9d especially nitinol. The use of this material has the advantage that the transmitted force also remains roughly the same at different adjustment distances.
The coupling rod and the coupling element can, however, also be connected to one another via a ball joint in order to achieve the above described optimum solid angle adjustment. Preferably the coupling arrangement is made and/or positioned in the implanted state such that the coupling end touches the coupling site without static prestress or with only slight prestress. This contributes to a reliable bond by surface adhesion when, at least in the implanted state, a film of moisture is formed between the coupling end and the coupling site. To do this, a natural film of moisture can be used which can be attributed to the 100% moisture which is present in the middle ear space.
The coupling end can advantageously be made concave before coupling with reference to the coupling site. In this way, when the coupling end is applied to the coupling site, a cavity is formed which is pressed together by slightly pressing against the coupling end. The negative pressure which has formed here supports adhesion.
The coupling site can also be formed by a coupling plate which can optionally be anatomically matched and which in the implanted state is securely joined, for example, cemented, to the surface of the part of the ossicular chain with which contact is to be made, the footplate of the stapes or a membrane which closes the round window or an artificial window. In this way, provisions can be made for an even better defined adhesion action because dynamic force transmission takes place between two defined materials and geometries which can be optimized for adhesion coupling. Coupling is then also more reproducible in its action than in direct action on an ossicle.
Improved anatomic adjustment possibilities can be achieved under certain circumstances by the coupling element having a large-area, open structure on the coupling end and/or being provided with several elastic arms.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention.